Import of assembled PTS1 proteins into peroxisomes of the yeast Hansenula polymorpha: yes and no!

Previously, Waterham et al. [EMBO J. 12 (1993) 4785] reported that cytosolic oligomeric alcohol oxidase (AO) is not incorporated into peroxisomes after reassembly of the organelles in the temperature-sensitive peroxisome-deficient mutant pex1-6(ts) of Hansenula polymorpha shifted to permissive growth conditions. Here, we show that the failure to import assembled AO protein is not exemplary for other folded proteins because both an artificial peroxisomal matrix protein, PTS1-tagged GFP (GFP.SKL), and the endogenous dimeric PTS1 protein dihydroxyacetone synthase (DHAS) were imported under identical conditions. In vitro receptor-ligand binding studies using immobilised H. polymorpha Pex5p and crude extracts of methanol-induced pex1-6(ts) cells, showed that AO octamers did not interact with the recombinant PTS1 receptor, at conditions that allowed binding of folded GFP.SKL and dimeric DHAS. This shows that import of oligomeric proteins is not a universal pathway for peroxisomal matrix proteins.